It has been known for some time to connect electric units, in particular electric hand tools or other working machines, to the mains, that is, the power source lines, via conventional phase control circuits to enable the speed, for example, of the unit to be controlled manually or to be maintained at a predetermined nominal value. In addition, it is also possible to design such conventional phase control circuits in such a manner that they provide a so-called smooth-starting feature when switching on the unit which prevents the generation of undesirable power-line load peaks which may occur in motor-driven units due to the high starting current and which may, for example, lead to fuse tripping.
Suitable phase control circuits are commercially available and are mostly constructed in the form of an integrated circuit (IC) offering a large number of external connections. Depending on the application for which it is intended, such a standard IC can be adapted as required, for example, used as a phase control by mounting additional external equipment, in accordance with the user's wishes, and in accordance with the connected electrical unit and the latter's requirements.
It has also been known to use an automatic starting system for switching on an auxiliary unit in combination with a main unit. Such an arrangement is described hereafter in more detail with reference to an electric hand tool producing, for example, dust or chips in combination with a vacuum cleaner intended for exhausting such chips. It goes, however, without saying that the invention described hereafter is by no means limited to this particular field of application.
It is highly desirable, and in fact usual practice in connection with the operation of certain electric hand tools, for example circular saws, planes, angle sanders, platten sanders, and basically in all cases where chips or dusts are produced in operation of these electric tools, to connect simultaneously an exhauster. The exhauster will then remove any particles occurring or produced during operation of the electric tool. Sometimes, such electric hand tools are already equipped with separate suction connections.
In order to ensure that such a vacuum cleaner or exhauster will operate only when the associated main unit is in operation, one has heretofore used an additional circuit by means of which the plug of the main unit, for example, can be fitted in, and connected to, a matching socket in the auxiliary unit. The auxiliary unit is then connected to the supply mains and feeds power to the auxiliary unit, in the present case the vacuum cleaner, and simultaneously controls the auxiliary unit. The auxiliary unit is provided with a current sensor, usually in the form of a transformer, for detecting the current absorbed by the main unit or external unit via the socket when the unit is started, and for supplying a signal to the IC of the phase control in the auxiliary unit. The auxiliary unit, or the IC of the phase control for the vacuum cleaner motor, then detects--by way of the current flowing to the main unit--that the latter has been started and causes the vacuum cleaner to start operating, too, in the way of an automatic starting control.
So, when an external unit, for example an angle sander with dust exhauster, is connected to the socket of the auxiliary unit, the dust exhauster will start operating, too, and exhaust any dust produced during the sanding operation. In this case, the operating current of the externally connected main unit flows through the primary winding of the transformer; the secondary current of the transformer is used in the IC of the phase control serving as automatic starting feature for triggering the latter's triac gate whereby the latter fires so that the dust exhauster starts operating, too.
However, such a known circuit numerous problems.
If an external unit with only small power consumption is connected, then one cannot exclude the possibility that the triac of the phase control may not receive sufficient ignition current, i.e. there is a risk that the triac may be damaged and fail due to so-called "hot spots" occurring as a result of this situation.
The simple approach to use a triac with sufficiently low ignition current does not, however, solve the problem as in the current-carrying condition of the triac, a voltage of approx. 1.5 to 2 V (to give some numerical values) is present at the triac gate, and energy is stored in this case in the secondary winding of the transformer even without any primary current. When in the zero current condition of the connected auxiliary unit, the triac is reset and no current flows on the primary side of the transformer from the main unit which when connected to the same mains (power source lines) then always assumes a zero current condition, the voltage present at the gate of the triac breaks down, too. However, this leads to the discharge of the energy stored in the transformer back into the gate and, accordingly, to repeated firing of the triac if the latter fires easily. This, too, may cause damage to the triac.
If, in contrast, a unit of high power is connected as an external unit (main unit) then the primary current and, accordingly, the secondary current, too, are normally so high that the current flowing into the gate of the triac exceeds by far the admissible maximum values which may again result in destruction of the triac.
Another problem connected with external units of high power lies in the fact that in the case of iron transformers using laminated stacks of sheets, the high primary current generates a field strength in the iron so high that the transformer will heat up heavily and may even be disconnected.
Now, it is the object of the present invention to remedy these disadvantages and to design a device ensuring automatic starting of an auxiliary unit when a main unit is switched on, so that, while external units of any desired power input can be connected, one still obtains at any time clear firing pulses for a triac controlling the auxiliary unit.